Squirrelpox virus (SQPV) and adenovirus produce pathological disease in native red squirrels (Sciurus vulgaris). SQPV in particular is a significant factor in regional population declines and is generally prevalent in the UK's introduced grey squirrel (Sciurus carolinensis) population as an asymptomatic infection. Despite the role of the grey squirrel as a virus reservoir and potential inter-specific infection pathways being highlighted, there remains a paucity of field study data with known relative inter-specific infection rates and quantified frequency of interactions. Intriguingly, whilst captive zoological red squirrel collections are often present within woodland habitat containing wild grey squirrels, clinical pox cases are rarely observed unless red squirrels are released from the enclosures. In 2011 we monitored grey squirrel activity on an enclosure containing red squirrels. Grey squirrels were present for a cumulative total of 47.5 minutes within the twenty four hours of observation. A range of behaviours were recorded including feeding, and instances where discarded food fell into the red squirrel enclosures below. We interpret the value of these observations in the context of published theories of viral transmission. The local grey squirrels were subsequently culled and tested for evidence of both historical and current SQPV and adenovirus infections. Polymerase Chain Reaction (PCR) assays did not amplify adenovirus DNA from grey squirrel blood samples, but positive results were recorded in faeces (3/18, 17%) and (10/18, 56%) in parallel spleen samples from the same animals. This variation in tissue specific detection rates suggests that previous long-term surveillance of adenovirus in wild grey squirrels focussing on blood samples may have significantly underestimated true infection rates. Enzyme-Linked Immunosorbent Assay (ELISA) tests revealed exposure to SQPV by antibody presence in 33% of the animals. Additionally, 22% of the animals contained detectable levels of both viruses. In parallel with laboratory and field studies in 2011, we collated historical unpublished reports and archived data from a range of UK squirrel collections and highlight some key cases of infection. We recommend that further behavioural and viral screening studies are focussed within scenarios where captive red squirrels are sympatric with wild grey squirrels.
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